Prior to being used in manufacturing, a raw polymerization reaction product, or raw polymer, typically undergoes various processing steps, collectively known as polymer finishing.
For example, FIG. 1 of U.S. Pat. No. 6,867,270 and the related detailed description illustrate one process for a polymerization conducted in a gas phase reactor to produce a raw polymer product. As shown in FIG. 1 from that patent, in gas phase reactors, polymerization may be conducted in a fluidized bed in which the reaction mixture (including a bed of polymer particles, catalyst, reactants and inert gases) may be maintained in a fluidized condition by the continuous upward flow of a fluidizing gas stream from the base of the gas phase reactor 75. Cycle gas may be taken from the top of the reactor 75 through line 135. The circulating cycle gas may be compressed by a compressor 145 and cooled by a heat exchanger 155 before being reintroduced at the base of the reactor 75 as a fluidizing gas stream. The fluidizing gas stream may also contain make-up reactants and inert gases, which can be introduced into line 135 by line 161 and/or line 160. As fresh polymer is produced, polymer product may be withdrawn by one or more discharge outlets 30 disposed in the lower portion of the reactor 75. The polymer product may be transferred into a product chamber 205 and then into a product blow tank 215 which enables the transfer of the polymer product through transfer line 25 into a product purge bin 200.
A similar polymerization process may also be conducted in a liquid phase reactor, such as a solution polymerization reactor. For example, a liquid phase polymerization reaction may be conducted in a fluidized bed in which the reaction mixture (including a bed of polymer particles, catalyst, reactants and solvent) may be maintained in a fluidized condition by the continuous flow of a fluidizing liquid stream. Following the polymerization, the solvent may be recovered from the polymerization product, for example, via flash, centrifuge, or other type of separation. Following separation from the solvent, the polymer product may be withdrawn from the reactor and transferred into a product purge bin, for storage and degassing of the polymer prior to further processing.
In either a liquid phase or a gas phase polymerization reaction system, nitrogen and/or steam may be injected into the purge bin in order to remove reactants and terminate or prevent any continuing polymerization. The polymer product, for example in form of a polymer powder, may then be transferred out of the product purge bin to downstream operations, which may include extrusion or packaging operations, and will be referred to herein as “polymer finishing.”
During polymer finishing, the resulting polymer from such liquid-phase or gas-phase polymerizations can be compounded with additives and/or blended with other polymers to form compositions that can then be used in articles of manufacture. For example, the compounded polymer blend can be extruded through a die to form pelletized polymer resin that can be further processed, such as via injection or blow molding. The additives can include antioxidants, nucleating agents, acid scavengers, plasticizers, stabilizers, anticorrosion agents, blowing agents, ultraviolet light absorbents, quenchers, antistatic agents, slip agents, pigments, dyes and fillers, and cure agents, such as peroxides, among others.
Solids handling is an integral part of the polymer finishing process. Conveyor belts have typically been used for transporting a polymer, for example in form of a polymer powder, from a purge bin to a mixer, where the polymer can be compounded with other polymers and additives. In order to produce polymer compounds consistently having the desired properties, the relative proportions of components to be compounded must be carefully measured, controlled and maintained.
A variety of devices have been used for weighing solids during polymer finishing, including specialized weighing bins, hoppers, and weighing conveyor belts. For example, U.S. Pat. No. 4,395,131 discloses use of a weighing hopper for measuring a weight of a solid, such as a polymer powder, where a screw conveyor may be used to feed the solid to a weighing hopper equipped with a weighing scale.
A weighing bin or hopper can typically provide an accurate measurement of polymer powder mass flow out of the bin in what is referred to as the loss-in-weight measurement. However, the weighing bin method alone does not provide a continuous measurement, as the weighing bin alternates between a filling and a draining stage. Also, the larger the size of the bin or the hopper being used, the lesser the accuracy of the loss-in-weight mass flow rate measurement, as the absolute value of the change in mass inside the bin or hopper may be several magnitudes smaller than the overall mass of the bin or hopper, and thus requiring a different measurement scale. Further, the mass flow rate of a polymer powder leaving the weighing bin, although accurate, may not be the true indication of the polymer flow rate through the downstream conveyor and into the mixer.
Another device for weighing a solid, such as disclosed in U.S. Pat. No. 6,523,721, includes a weighing conveyor belt system having a weighing device, such as a load cell, for weighing the amount of solid material on the conveyor belt. Using the mass of the polymer powder on the conveyor belt, the length of the conveyor belt along which the mass is measured, and the speed of the conveyor belt, the mass flow rate of the polymer powder may be determined However, uneven distribution of the polymer powder on a weighing conveyor belt and unintended movement of the polymer powder on the conveyor belt due to dusting, slipping, or tracking can significantly impact the mass flow rate measurement accuracy of the weighing belt conveyor.
Further, any inaccuracy and/or inconsistency in a mass flow rate measurement of the polymer powder may result in poor controlling of the mass flow rates and/or the mass flow rate ratios of the polymer powder and the other polymers and/or additives, which in turn may result in poor qualify and/or consistency of the finished polymer product.
Accordingly, there exists a need for systems and methods for polymer finishing, including more robust devices for measuring and controlling the mass flow rate of a solid polymer material for mixing with other polymers and/or additives, in order to improve the reliability and accuracy of the equipment and the consistency of the resulting polymer product.